Hard to say about the best season. For OSU's part, 2002 and 2014 were otherwordly. There were not a ton of expectations in 2002, and Maurice Clarett was often injured, leading to extremely close wins. This was also back when Michigan was more of a scary monster, so actually beating them to go to the national championship game was monumental. And of course very few gave OSU any sort of chance to beat Miami, and the game itself is on the short list for best games of all time (and I was there), so for my money it will always be the best season. I was also a 2L at OSU law at the time, so I'm most definitely biased.
2014 was similar, and also very different. There were expectations until Braxton Miller was a no go for the season and OSU had to turn to a freshman JT Barrett, and they promptly lose to Virginia Tech. Then they somewhat quietly dominated most of the rest of their schedule (with a notable exception at Penn State). Then just as they were getting back on the national radar for a playoff spot, JT broke his leg v. Michigan, which really took the air out of the whole state. Then they obliterated Wisconsin and got to the playoffs (the very first playoff). They were underdogs against Alabama, and got an epic win, before finishing off the season in a pretty ugly and uneventful championship game against Oregon.
The reason I write all that is because there's a comparison to be made. OSU was was a championship contender coming into the season, they were not world beaters, and really didn't play a truly impressive game until the last one against Michigan. As far as single days go, that was one of the best I've had, but as a season, not so much. Of course, if they sneak into the playoffs, beat an alltime great like Bama and win a championship, things are different.
For my money, right now, Northwestern is the team. Pretty mild expectations, a mostly brutal start to the season, and they kick ass and win their division pretty handily. Now they are playing for the Rose Bowl.